The present invention relates generally to the processing of hydrocarbon gas streams to recover a desired condensable portion thereof, and, in particular, to an improved cryogenic gas separation process to recover propane and heavier hydrocarbons from a hydrocarbon gas stream such as natural gas or similar refinery or process gas streams.
The market demand and price for propane and heavier hydrocarbon liquids has remained such that it is most always economical to recover these compounds from natural gas or similar refinery or process streams. However, this has not been the case for ethane. The demand and price for ethane has fluctuated tremendously such that there are extended periods when it is ore economical to leave the ethane in fuel gas and only recover the propane and heavier hydrocarbons. Also, there are market locations where it may never be economical to recover ethane. Thus, there is a demand for processes which recover propane and heavier hydrocarbons from a hydrocarbon gas stream, while rejecting the ethane and lighter components to the residue or fuel gas.
Several variations of prior art cryogenic hydrocarbon separation processes are disclosed in U.S. Pat. Nos. 4,140,504; 4,157,904; and 4,278,457. In U.S. Pat. No. 4,157,904, for example, a process is disclosed for cryogenically separating a hydrocarbon gas stream to recover ethane and heavier hydrocarbons. As disclosed therein, the gas to be separated is precooled at a high pressure of about 900 psia to cause partial condensation. The condensate liquid and vapor are then separated. A first portion of the vapor is expanded to a lower pressure and fed to the demethanizer column. A second portion of the vapor is first cooled by passing in heat exchange relationship with the residue gas from the demethanizer column and then passed through an expansion valve to the demethanizer column at locations either above or below the point of injection of the first portion of expanded vapor depending upon the exact temperature conditions. The condensate from the separator is further cooled and then split into two streams. One stream is expanded to a lower pressure and passed to a lower region of the demethanizer, while the other stream is directly mixed with the second portion of vapor from the separator prior to further cooling and expansion. Ethane recovery is said to be on the order of 90% or better, with propane recovery said to be on the order of 98%.
As most of the prior art processes described in the aforementioned patents, and in particular the aforedescribed process, are designed primarily for high ethane recovery, as opposed to ethane rejection, these prior artprocesses are believed not suitable without modification to efficiently recover propane and heavy hydrocarbons from a mixed hydrocarbon gas stream while rejecting lighter hydrocarbon liquids, in particular ethane.